Since "builtin" terminfo definitions were implemented (7cbf52db1b),
the decisions made by tui.c and terminfo.c are more relevant. Exposing
that decision in the 'term' option helps with troubleshooting.
Also: remove code that allowed setting t_Co. `:set t_Co=…` has never
worked; the highlight_spec test asserting that nvim_set_option('t_Co')
_does_ work makes no sense, and should not have worked.
This fixes an apparent difference in behavior between Lua and LuaJIT.
Lua fails to format nil:
test/functional/terminal/tui_spec.lua:381: bad argument #2 to 'format' (string expected, got nil)
To deal with SIGWINCH limitations on Windows, change some resize tests
to _shrink_ the screen width. ... But this didn't work, so still
ignoring those tests on Windows.
The terminfo entry for linux only advertises 8 colours, but nvim tries
to make it display 16 colours anyway, resulting in erroneous SGR control
sequences for colours 8 and above. The Linux kernel terminal emulator
itself has actually understood the 256-colour control sequences since
version 4.8 and the 16-colour control sequences since version 4.9. Thus
we apply the same terminfo fixup as we apply for *xterm* and *256*, to
emit the 16-colour and 256-colour control sequences even if terminfo's
setaf and setab do not advertise them.
Calling cmd.exe in Windows follows a very different pattern from Vim.
The primary difference is that Vim does a nested call to cmd.exe, e.g.
the following call in Vim
system('echo a 2>&1')
spawns the following processes
"C:\Program Files (x86)\Vim\vim80\vimrun" -s C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
^>C:\Users\dummy\AppData\Local\Temp\VIoC169.tmp 2^>^&1)
C:\Windows\system32\cmd.exe /c C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
^>C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2^>^&1)
C:\Windows\system32\cmd.exe /c (echo a 2>&1
>C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2>&1)
The escaping with ^ is needed because cmd.exe calls itself and needs to
preserve the special metacharacters for the last call. However in nvim
no nested call is made, system('') spawns a single cmd.exe process.
Setting shellxescape to "" disables escaping with ^.
The previous default for shellxquote=( wrapped any command in
parenthesis, in Vim this is more meaningful due to the use of tempfiles
to store the output and redirection (also see &shellquote). There is
a slight benefit in having the default be empty because some expressions
that run in console will not run within parens e.g. due to unbalanced
double quotes
system('echo "a b')
